Hard surface emulation is the ability of a manipulator to simulate a rigid virtual surface of relatively arbitrary shape. The use of such a surface implies projecting a virtual interface into a separate environment or workspace. As such, a user is able to move freely within this workspace until they come into contact with the interface. The most basic application of this concept is the use of a physical template, such as a ruler on a piece of paper.
Hard surface emulation in the context of assisting users to perform precision motion control tasks has a wide array of potential applications, ranging from large industrial part handling tasks to surgical procedures. Humans are not endowed with the high repeatability, precision or stability of robots. They are, however much better adapted to decision making and strategic planning in variable environments and in controlling physical interactions, such as those involved in using various tools. Haptic interfaces can be used to merge these distinct abilities.
To date, virtually all haptic research on stiff wall emulation has focused on impedance or admittance-generating algorithms. These algorithms are used to determine the forces or displacements required by the haptic architecture to emulate the virtual environment. Hard surfaces are typically approximated as a spring of given stiffness and hence require actual penetration of the virtual surface to activate the restoring forces. Due to hardware and software limitations, such as response lag time, joint backlash, structural flex, sensor noise, etc, systems based on this concept are not capable of rendering truly hard surfaces or of handling large or sustained user forces without causing instability or lack of precision. These requirements are, however, essential if one wishes to use haptic force feedback in guidance and region-restriction tasks.
A number of new concepts for emulating hard surfaces based both on modified control algorithms as well as on new mechanical concepts have been developed in order to resolve these challenges. One such concept is described in US patent application publication No. 2004/0128026A1, which is hereby incorporated by reference in its entirety. It has been applied in the form of a haptic robot, named Acrobot, and used in cutting tool guidance during total and unicompartmental knee arthroplasties. The concept introduces a region of increasing robot stiffness at the boundary of the free-motion and restricted regions. Within this region, the robot impedance increases and the admittance decreases based on the current location of the tool with respect to the restricted boundary. The purpose of Region II is to provide a smoother transition between the free motion region and the restricted region, thus preventing instability and decreasing the possibility of surface penetration due to delays in the control loop. The drawbacks are springiness at the boundary, vibrating motion at an inclined boundary and restricted motion along the boundary due to the increased impedance in this region. Some practical drawbacks are that a force transducer is required on the interface between the user and the device. Additionally, the structural architecture and motors must be able to provide sufficient impedance to the user, requiring large parts. This also creates significant friction in the system, requiring motion assistance from the robot to emulate uninhibited motion in the free region. All of this results in a relatively costly robot.
Another concept is described in U.S. Pat. No. 5,952,796, which is hereby incorporated by reference in its entirety, and is based on a continually variable transmission (CVT) concept. A CVT device is strictly defined as one having a continuous range of transmission ratios, independent of the amount of torque being applied to it. The drawbacks of this concept are that it necessarily requires force sensors to keep track of user intentions. Depending on the task, it can also result in rather bulky architectures with large amounts of inertia. More importantly, the inherent characteristic of the design in which the wheel steers continuously rather than in discrete steps, causes a sense of hesitation when the user rapidly pushes the device from rest and surface penetration when the device approaches a boundary at a high angle.
A third concept uses a double freewheel and motor combination that allows passive motion within a set of dynamic constraints, as described in U.S. Pat. No. 5,529,159, which is hereby incorporated by reference in its entirety. A freely rotating shaft is constrained by a freewheel to the rotational speed of a second parallel shaft driven by a motor. The concept allows the control of relative motion between two serial manipulator arms, each connected to one of the shafts. Two drawbacks with the design include low stiffness of the system and jagged motion in certain regions during path or surface following.
A fourth concept, called PTER and described in U.S. Pat. No. 5,704,253, which is hereby incorporated by reference in its entirety, is based on the use of clutches and brakes to regulate the relative rotational velocity between two manipulator links. The primary disadvantages of this system are penetration of the hard surface and smoothness during path-following tasks.